Color Rolling is a slow change of the image color when the video is being taken under the illumination of lamps that are powered by a fluctuating power supply and in particular with lamps such as fluorescent lamp. By fluctuating power supply is meant power supply that may be changed periodically. The lamp may be a fluorescent lamp. The light of such lamps may be powered by AC power line, provides illumination that changes with a periodicity twice faster than the power line. On the other hand, video camera operates at a frequency that is very close to that of the power line. The operation of the video camera is typically not synchronized with the power line. The difference between the sinusoid of the power line and the sinusoid of the lamp light illumination may cause a slow varying change in the illumination that is seen by the camera when sequential frames are captured. The difference between the sinusoid of the power line and the sinusoid of the lamp is termed herein as a beating effect. This change in the illumination may be seen both in the luminance and in the chrominance domains. In the luminance domain, the difference appears as if the output signal level of the camera is fluctuating slowly, increasing and decreasing periodically. In the chrominance domain, the difference produces artificially looking colors with tones that are circling through the gamut.
Color rolling is especially significant in cameras that do not control automatically the lens iris, or operate in the Wide Dynamic Range (WDR) mode. Typically when the camera do not control automatically the iris, its exposure control is done thorough the electronic shutter that controls the exposure time. If the lens are fully opened then the shutter becomes small. The opening time is lower than one full period of the fluorescent lighting illumination. Similarly, in the WDR camera, the Short channel is set to make short time exposure to cope with brightly lit portions of the scene, so the exposure is also much smaller than the fluorescent period. FIG. 1 shows the differences in the strength of the output image as a result of the short time exposure. Since the camera exposure 101 is not synchronized with the light illumination 102, the strength of the output image 103, and the color may change throughout the time.
A long exposure may cover almost the whole lighting period and thus may reduce the changes in the output image because the same intensity is observed during all the frames. FIG. 2 shows that the output image 203 is stable when the camera exposure 202 is long and substantially covers the whole light illumination period 201. However, a long exposure may not be adequate when there is plenty of light.
Method known in the art, which are typically used in most cameras today, provide a partial solution to the color-rolling problem. Typically, some method of color rolling state detection is employed, either by detecting periodic color fluctuations in some predefined color space, or by detecting periodical luminance changes, indicating the presence of the periodical lighting. Based on the detection of color rolling phenomena, one, or a combination of common solutions is used. The first solution is to reduce significantly the color of the Short channel in case of WDR camera or the color of the Normal channel in case of normal camera with shutter that is less than one full period of fluorescent lighting. Such a solution may cause the color level to be low and thus may cause the whole frame to look pail and not vivid. The second solution attempts to find a proper White Balance matrix that reduces the color rolling color phenomena while still keeping reasonable colors in the areas not strongly affected by the fluorescent light. In cases where there is only one color matrix for both Long and Short channels such as usually the case in WDR camera, changing the matrix that resolves color rolling in the short channel produces an artificial color rolling in the long channel.
Another method known in the art can be used only when the camera is connected to an AC supply. In such a case, the operation of the camera may be synchronized with the AC supply as a result of measuring the frequency of the AC supply. Such synchronization is termed herein as Line-lock. Such synchronization causes the operation of the camera to be matched to the lamp light, cycle, phase or a combination thereof.